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Thursday, November 20, 2008

Embankment dam

An embankment dam is a massive artificial water barrier. It is typically created by the emplacement and compaction of a complex semi-plastic mound of various compositions of soil, sand, clay and/or rock. It has a semi-permanent waterproof natural covering for its surface, and a dense, waterproof core. This makes such a dam impervious to surface or seepage erosion. The force of the impoundment creates a downward thrust upon the mass of the dam, greatly increasing the weight of the dam on its foundation. This added force effectively seals and makes waterproof the underlying foundation of the dam, at the interface between the dam and its stream bed. Such a dam is composed of fragmented independent material particles. The friction and interaction of particles binds the particles together into a stable mass rather than the use of a cementing substance.


Embankment dams come in two types: the earth-filled dam (also called an earthen dam or terrain dam) made of compacted earth, and the rock-filled dam. A cross-section of an embankment dam shows a shape like a bank, or hill. Most have a central section or core composed of an impermeable material to stop water from seeping through the dam. The core can be of clay, concrete or asphalt concrete. This dam type is a good choice for sites with wide valleys. Since they exert little pressure on their foundations, they can be built on hard rock or softer soils. For a rockfill dam, rockfill is blasted using explosives to break the rock. Additionally. the rock pieces may need to be crushed into smaller chunks to get the right range of size for use in an embankment dam.


The building of a dam and the filling of the reservoir behind it places a new weight on the floor and sides of a valley. The stress of the water increases linearly with its depth. Water also pushes against the upstream face of the dam, a nonrigid structure that under stress behaves semiplastically, and causes greater need for adjustment (flexibility) near the base of the dam than at shallower water levels. Thus the stress level of the dam must be calculated in advance of building to ensure that its break level threshold is not exceeded.

Overtopping or overflow of an embankment dam outside of its spillways will cause disastrous flooding through the eventual failure of the dam. In the failure process the sustained hydraulic force and pressure caused by an overtopping surface runoff; immediately erodes the dam's material structure as it flows over the top of the dam. Even a small sustained overtopping surface flow can remove thousands of tons of overburden soil from the mass of the dam within hours. The removal of this mass unbalances the forces that stabilize the dam against its impoundment. The mass of water still impounded behind the dam presses against the lighter mass of the embankment, (made lighter by surface erosion). As the mass of the dam gets lighter, the impoundment begins to move the whole structure. The embankment, having almost no elastic strength, begins to break into separate pieces, naturally allowing the impounded water to flow between them eroding and removing more material as it goes. In the final stages of failure the remaining pieces of the embankment offer almost no resistance to the flow of the water; as they continue to fracture into smaller and smaller sections of earth and/or rock. The overtopped earth embankment dam literally disolves into a thick mud soup of earth, rocks and water.

Therefore safety requirements for the spillway are high, requiring the spillway to be capable of containing a maximum flood stage. Specifying a spillway able to contain a five hundred year flood is common.

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